For me Osanai feels like a main suspect, especially after that train sound in the phone but I doubt she would do something so careless.

Also the whole thing with Kobato’s girlfriend feels like a conversation before breakup. And she’s been acting suspicious but then it’s just probably her having another affair.

The friend of that red-haired guy seems like someone who’s not just your typical nobody… He’s constantly pushing the red guy in the right direction.

Honestly I can’t wait to see what Kobato had planned. He has way less information than the viewer and as a viewer I can’t say anything for sure.